There are a number of high pressure die casting devices/machines in use but many produce cast articles which have a high degree of porosity. If the level of porosity is allowed to rise above certain limits, the pores can adversely affect the properties of the component so as to lead to the failure or deterioration of the cast article during use (most porosity remains internal; only if the die castings are machined is the porosity usually exposed). Therefore, in any high pressure die casting device, an object will be to reduce the porosity levels as close to zero as possible.
A high pressure die casting (hpdc) machine which is used extensively is the "cold chamber" hpdc machine which manufactures aluminium alloys. This machine operates by transferring molten metal from a shot sleeve into a die cavity by means of a high velocity piston or plunger. The molten metal is forced along a series of channels or a runner system and through a fixed, narrow gate or opening into the die cavity. The liquid metal is then effectively sprayed through the gate to produce a first coating over the surface of the die cavity and then the remainder of the liquid metal is introduced into the cavity to complete the cast article. The first coating of liquid metal commonly produces a very fine grain surface layer having a very smooth surface finish. However, the cold chamber hpdc process suffers from two major disadvantages. First, because the molten metal has to flow through a runner system of channels, its temperature will fall by the time it reaches the narrow gate and consequently, it will freeze around the narrow gate which reduces the pressure which can be transmitted effectively from the runner and gate onto the metal in the die cavity. The reduction in pressure transmission will produce die castings which are notoriously porous and may, therefore, not be heat treatable for fear of blistering. Furthermore, subsequent machining operations will expose the porosity which causes a high rejection rate. Secondly, the cold chamber hpdc process is commonly only about 33% efficient because approximately 50% of the cast metal (i.e in the runner and gate sections) needs to be removed from each casting for remelting. Moreover, since there is an additional casting scrap rate of 5-15% the efficiency of the cold chamber hpdc process is rarely greater than 25% in material utilisation and considerably less than 20% in energy utilisation.
Several solutions have been proposed to reduce porosity. One such solution involves evacuating the die set prior to casting with a view to reducing gas entrapment. However, the casting still freezes or solidifies remote from the point of application of pressure and therefore, the solidifying casting cannot be fed from the reservoir of metal in the runner and wad. Hence, contraction cavities arise in the casting. A further solution proposed was to purge the die set with oxygen or another suitable gas which would combine spontaneously with the liquid metal to remove gas from the die set. However, contraction cavities are still formed. The mould is sprayed with a lubricant prior to the casting which evaporates on contact with the hot metal so that gases are still present in the mould. A slightly different approach was to apply enhanced pressure on the wad by a smaller secondary piston but porosity still exists in the casting due to remote application of the pressure and freezing off at the gate.
In order to attempt to limit the amount of porosity, another approach to high pressure die casting has been devised which is more accurately described as a "squeeze" casting. Squeeze casting is the term used to denote processes in which liquid metal is solidified under the action of a high external pressure. Two different types of squeeze casting technology have evolved based upon different approaches to metal metering and metal movement and also upon the manner in which the pressure is applied to the metal in the mould. These two processes have been given the names "direct" and "indirect" squeeze casting.
In direct squeeze casting, the die set is a split mould consisting of a lower female cavity and an upper male punch. Sufficient pressure is applied to the punch, which moves to compress the liquid/solid mixture during freezing to suppress the appearance of either gas porosity or shrinkage porosity in the casting. Direct squeeze casting is thus a hybrid process combining gravity die casting with closed die forging.
In indirect squeeze casting, liquid metal is injected into a closed die cavity by a small diameter piston, by which mechanism the pressure is also applied during freezing. Squeeze pressures are limited by the size of the piston and, for large area castings, some thin sections of the casting may freeze off locally and prevent the transmission of pressure to remoter regions thus allowing porosity to form. The current art of indirect squeeze casting uses vertical injection of liquid metal into the die set which has either a vertical or horizontal opening.
It is to be appreciated that squeeze casting can produce much lower levels of porosity than high pressure die casting and therefore, a combination of both types of casting would be desirable. In hpdc (usually horizontal machines) the wad and runners are the only parts which are pressurised to a maximum extent because the gate freezes or solidifies and then the metal in the die freezes under low pressure. In indirect squeeze casting (vertical or horizontal machines), the same is also true but to a lesser extent because the gates are wide open and are of fixed geometry.